A 24-hour fast means skipping all food for a full day while staying hydrated with water and other zero-calorie drinks. It’s one of the simpler forms of intermittent fasting, and most healthy adults can do it safely with a bit of planning. Here’s how to set yourself up, what to expect hour by hour, and how to break your fast without feeling terrible.
Pick Your Fasting Window
The easiest approach is to fast from one meal to the same meal the next day. Most people choose dinner-to-dinner: eat a normal dinner at, say, 7 p.m., then don’t eat again until 7 p.m. the following day. This way you sleep through a large chunk of the fast and only have to get through one full waking day without food. A lunch-to-lunch schedule works too, and some people prefer it because they go to bed having eaten and wake up already past the halfway point.
Whichever window you choose, plan your fast on a day when you’re moderately busy but not doing anything physically intense. A regular workday often works well. Sitting at home with nothing to do tends to make hunger harder to ignore, while heavy manual labor or long gym sessions on an empty stomach can leave you lightheaded.
What You Can Drink
Water is your primary drink, and you should have plenty of it. Black coffee and plain, unsweetened tea are also fine. Both are zero-calorie and won’t interrupt the metabolic processes that make fasting useful. Avoid adding milk, cream, sugar, or honey, as even small amounts of calories can blunt some of the hormonal shifts that happen during a fast. Sparkling water and herbal teas are also good options for variety.
One thing many people overlook is electrolytes. When you fast, your insulin drops and your kidneys flush out more sodium than usual. This sodium loss is the main reason people get headaches, fatigue, and brain fog during a fast. Adding a pinch of salt to your water (roughly a quarter teaspoon per glass, sipped throughout the day) can prevent most of these symptoms. You can also use an electrolyte powder or drink salted broth if you prefer, though broth does contain a small number of calories.
What Happens in Your Body, Hour by Hour
For the first several hours, your body runs on the glucose circulating in your blood and the glycogen stored in your liver. This is business as usual. Around the 10-hour mark, those glycogen stores run low, and your body begins shifting to fat as its primary fuel source. This transition is sometimes called the “metabolic switch,” and it’s when many of the benefits of fasting start to kick in.
By the end of a 24-hour fast, several notable things have happened. Growth hormone levels rise substantially. One study found that during a 24-hour water-only fast, growth hormone increased roughly 5-fold in men and up to 14-fold in women. People who started with very low baseline levels saw the most dramatic jumps, with a median increase of over 1,200%. Growth hormone helps preserve muscle tissue and supports fat burning, which is part of why fasting doesn’t cause as much muscle loss as you might expect.
Your cells also ramp up a recycling process where they break down damaged components and repurpose the raw materials. Research in mice shows this process increases measurably in liver and brain cells within the first 24 hours of fasting and peaks around 48 hours. In practical terms, this is one of the mechanisms behind fasting’s potential benefits for cellular health and longevity.
Managing Hunger Through the Day
Hunger during a fast doesn’t build in a straight line. It comes in waves that align with your normal meal times. If you usually eat breakfast, lunch, and dinner, expect to feel the strongest pull around those hours. The key insight is that these waves pass. Ghrelin, the hormone that triggers hunger, spikes before your habitual meal times and then drops again whether or not you eat. Most people find that the urge to eat peaks somewhere between hours 12 and 18, then actually eases as they approach the 24-hour mark.
Interestingly, research shows that total ghrelin levels don’t actually increase overall after 24 hours of fasting. Individual responses vary widely, with some people’s ghrelin rising by several hundred units and others’ dropping by the same amount. This explains why some people find their first 24-hour fast surprisingly easy while others struggle more. If you’re in the latter group, keeping busy, sipping on coffee or tea, and reminding yourself that each hunger wave lasts only 20 to 30 minutes can make a real difference.
How to Break Your Fast
What you eat when the 24 hours are up matters more than you might think. Your digestive system has been resting, and hitting it with a greasy burger, a pile of raw vegetables, or a sugary dessert can cause bloating, cramping, and discomfort. High-fat, high-sugar, and high-fiber foods are the hardest on your gut after a fast.
Start with something small and easy to digest. Good options include:
- Eggs or avocado, which provide protein and healthy fats without overwhelming your system
- A simple soup with protein and soft carbs, like lentil soup or a broth-based soup with tofu or pasta
- Unsweetened yogurt or kefir, which are fermented and gentle on digestion
Eat slowly, give yourself 30 to 60 minutes, and then have a fuller meal if you’re still hungry. Most people find that after a 24-hour fast, their appetite is smaller than expected, and they naturally eat less than they would after a day of normal eating. Resist the urge to “make up for lost time” by overeating. That’s the fastest route to stomach pain and negating the calorie deficit you just created.
Who Should Skip a 24-Hour Fast
A 24-hour fast isn’t appropriate for everyone. People with diabetes, particularly those taking insulin or other blood sugar-lowering medications, face real risks of dangerous blood sugar drops. If you take blood pressure or heart medications, fasting can throw off your sodium and potassium balance in ways that matter. Anyone who takes medication that needs to be taken with food to avoid nausea or stomach irritation will have an obvious problem.
The NIH also flags several other groups who should avoid fasting or get medical guidance first: people under 25, anyone who is pregnant or breastfeeding, people with seizure disorders, night-shift workers, and anyone who operates heavy machinery at work. If you’re already at a low body weight, losing more can weaken your bones, suppress your immune system, and drain your energy levels.
Making It Easier the First Time
If you’ve never fasted for a full day, a few practical steps make the experience much smoother. First, eat a balanced meal with protein, fat, and complex carbs before you start. A dinner of salmon with roasted vegetables and rice, for example, will keep you satiated longer than a bowl of pasta with bread. Second, get your drinks ready the night before: fill a water bottle, set out your tea bags or coffee, and have some salt or electrolyte packets on hand.
During the fast itself, light activity helps. A walk, gentle yoga, or routine errands keep your mind off food without taxing your body. Avoid intense workouts, especially if this is your first fast. Some people report feeling sharper mentally in the later hours of a fast, which may be related to short-term increases in a protein called brain-derived neurotrophic factor (BDNF) in brain regions involved in memory and learning. Whether or not you notice a mental boost, most people find the afternoon easier than the morning if they stay occupied.
Finally, don’t fast on consecutive days when you’re starting out. Once a week or once every two weeks gives your body time to adapt and lets you learn how you personally respond. Some people take to it immediately. Others need two or three attempts before it feels routine. Both responses are normal.